U.S. Pat. No. 4,115,110 discloses the treatment of vanadium-containing materials such as vanadium-containing ore and scrap materials low in oxidic vanadium, for example, containing approximately 1 percent by weight of oxidic vanadium compounds. The material is subjected to a first roasting at about 600.degree. C., and it is then roasted a second time at a temperature of approximately 800.degree. C. Alkali compounds conventionally used in "salt roast" conversion are specifically not to be added according to the process of U.S. Pat. No. 4,115,110.
In the case of a high or oxidic vanadium compound content, the first roasting step is carried out in an atmosphere free of oxygen, compare Austrian Pat. No. 290,862. This requires special precautions, however, to preclude the access of oxygen. In general terms, the known techniques are not fully suited to recover vanadium from source materials having a high content of vanadium oxide or oxidic vanadium compounds. The prior art has only suggested processes to recover vanadium from vanadium-containing source materials with a greater content of oxidic vanadium compounds which processes are different from the method of the invention.
Thus, it has also been proposed in the prior art to combine the source material with additives, namely alkali metal compounds in the form of carbonates, chlorides, sulfates, nitrates and mixtures thereof, followed by grinding, and roasting. The roasted material is then leached, compare German Pat. No. 847,808.
The vanadium can be present in the source material in various valence states and in various states of bonding, for example as spinel-type FeO.V.sub.2 O.sub.3. Recovery of the vanadium values is carried out in the prior art processes by steps including: grinding the source material and adding the mentioned alkali additives. Roasting is carried out at a temperature of approximately 800.degree. C., usually in multiple hearth furnaces. The vanadium values are liberated and oxidized by atmospheric oxygen to the pentavalent state, according to the following equation: EQU V.sub.2 O.sub.3 +O.sub.2 =V.sub.2 O.sub.5 ( 1)
Reaction with the alkali additives yields sodium vanadate which is soluble in water, for example according to the following examples: EQU V.sub.2 O.sub.5 +Na.sub.2 CO.sub.3 =2NaVO.sub.3 +CO.sub.2 ( 2) EQU V.sub.2 O.sub.5 +Na.sub.2 SO.sub.4 =2NaVO.sub.3 +SO.sub.3 ( 3)
This sodium vanadate product is subsequently solubilized and extracted by leaching from the calcine. The leach solutions containing the vanadium values are subsequently further subjected to hydrometallurgical and chemical techniques to recover the vanadium compounds, i.e. as FeVO.sub.4, as is known in the art.
Unfortunately in such prior art processing source materials containing 6 percent or more by weight vanadium can not be directly subjected to an alkali roast. This is so because the necessary addition of large amounts of the mentioned additives would cause undesirable melting and consequent damage or obstruction of the equipment when the feed mixture is passed to the roasting step.
Addition of inert substances, for example leach residues from the processing cycle to preclude melting, will unduly dilute the source material. The practical and economic consequences of such dilution with respect to the processing are of considerable detriment, namely, reduced capacity for the roasting step, higher energy consumption for drying and heating of the extraneous material, reduced capacity for the leaching step, and an increased consumption of reagents.